05 groundwater contamination assessment in the al-quwy'yia
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precipitation of \100 mm while the annual potential
evapotranspiration exceeds 3,000 mm (Al-Saleh 1992).
The main source of water supply for the whole area is
groundwater either from shallow or deep aquifers
depending on the location and depth of the productive
wells. The groundwater is generally used for domestic
purposes, but there is also lack of knowledge concerning
water quality as in some cases the local Bedouin drink theextracted groundwater directly from the wells.
To highlight this contamination problem and to put in
place low cost method for tracing the pollutants in the
subsurface, geophysical surveys utilizing geoelectrical
techniques have been applied (Asfahani 2007; Yadav and
Singh 2007; Porsani et al. 2012). Time-domain electro-
magnetic and electrical resistivity methods are the pre-
ferred geoelectrical techniques and have wide applications
in delineating the shallow subsurface properties and tracing
pollution plumes (Pellerin 2002; Delgado Rodrı́guez et al.
2012). Both methods measure the contrast in electrical
conductivity properties of the subsurface rock units, whichvary according to rock type, water content, water quality
and temperature (Parasnis 1997). In the same rock units,
electrical conductivity is mainly controlled by the water
content and its quality within the formation matrix rather
than by the solid granular material itself. An increase in ion
dissemination in the groundwater due to pollution has a
direct effect on the groundwater electrical conductivity
properties (Metwaly et al. 2012a). This fact has been well
studied using an empirical relationship of Waxman and
Smits (1968), which suggested that as long as there are free
ions in the groundwater electrical conductivity will
increase. Transient electromagnetic method (TEM) and 2D
electrical resistivity tomography (2D ERT) data sets have
been utilized as fast, low cost and effective ways for
tracing low resistivity zones in the subsurface, which are
coincident with contaminated groundwater. Moreover, in
the present study, the location of the basement rocks, which
Fig. 1 Location map of the
dump site and the different
acquired data sets at Al-
Quwy’yia area. st # is the TEM
stations, v # is the VES station,
QAP2-1 is the borehole in the
study area and Qpro # is the 2D
ERT profiles
Fig. 2 Photos for the Kuff Fm. outcrop (a) and surface bond of the
wasted water at the dump site (b) Al-Quwy’yia area
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is dipping away toward the eastern part of the whole area,
may be traced using the two data sets.
Geological and hydrogeological setting
Until now, few geophysical studies have been carried out
along Al-Quwy’yia environs for the purpose of shallowsubsurface groundwater evaluation (Metwaly et al. 2012b).
Most of the works are on the geological and mineral
resources as the area is situated very close to the eastern
part of the Arabian shield (Senalp and Al-Duaiji 2001). Al-
Quwy’yia area is located close to the Arabian Shield,
which is composed of a basement complex bordering the
western side of the study area (Fig. 1). To the eastern side,
the sedimentary units overlie the basement rocks (Fig. 2a).
At the surface, there are weathering products (alluvium and
eolian deposits), which cover most of the area. Underlying
the weathering sediments, the lithological unit is composed
of Khuff limestone, which overlies the basement rocks(Fig. 2a). The shallow aquifer is exposed at the surface and
hydraulically extends to the Khuff limestone. The shallow
aquifer is recharged from the sparse rainfall over the
basement and limestone exposures (Nebert 1970).
Different types of land-usage are recognized within the
study area. The central part is represented by Al-Quwy’yia
City (urban built-up and rural areas), while the western and
southwestern parts are represented by basement complex
rocks. The Holocene alluvial deposits fill the wadi courses
within the study area. These wadi fill deposits have good
hydraulic characteristics, enhancing the groundwater
recharge as well as infiltrating the waste water originating
from either septic tanks or dump sites. The water table
contour map for the shallow Khuff aquifer is based on
measuring the depth to the water at 22 wells over the whole
Al-Quwy’yia area (Fig. 3). The general trend of ground-water flow is from the west and the southwest towards the
east and the northeast directions. These flow directions are
coincident with regional topographic trends.
Methodology and data acquisition
The time-domain electromagnetic method (TEM) and 2D
electrical resistivity methods (ERT) are well known in
exploration geophysics (Telford et al. 1995; Nabighian and
Macnae 1991). The methods have been applied together in
many ways making them ideal partners for shallowexploration. Although both methods measure the electrical
conductivity or resistivity of the subsurface, they sample
different volumes and have different degrees of sensitivity.
TEM is an inductive technique and has an area of inves-
tigation that is a function of the descending and expanding
image of the transmitted current. This area is typically
40 m 9 40 m or greater. The resistivity method is a gal-
vanic technique that samples a linear portion of the ground
Fig. 3 Water table contour map
of Khuff aquifer in Al-Quwy’yia area (2012)
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as defined by the area of current flow. The TEM method
gives an absolute measurement of the subsurface resistivity
while the electrical resistivity method gives a relative
measure of this quantity (Auken et al. 2001).
In this work, the TEM data sets have been acquired
using the TEM FAST 48HPC system with a single trans-
mitter and receiver loop (coincident loop) of length
50 m 9 50 m (AEMR 2007). The time of measurementranged from 4 to 16 ms including 48 time windows with a
repeatability frequency changing from 3.2 kHz to 11 Hz.
More details about the TEM FAST 48HPC system can be
found in the operating manual. The principles of the
TDEM method are described in Barsukov et al. (2006). 2D
electrical resistivity surveys are commonly used for shal-
low subsurface investigations particularly environmental
surveys (Loke 1999). Because of its efficiency and effec-
tiveness in producing images of the subsurface, the 2D
geoelectrical resistivity imaging actually measures the
apparent resistivity of the subsurface, which can be
inverted to develop a model of the subsurface structure andstratigraphy in terms of its electrical properties (Loke
2003). The resistivity of the subsurface is affected by
porosity, amount of water, ionic concentration of the pore
fluid and composition of the subsurface materials. There-
fore, the resistivity data can be used to identify, delineate
and map subsurface features such as electrically conductive
contamination plumes (Dawson et al. 2002). The acquired
2D data sets in this work have been collected using the
SYSCAL PRO system, which features 72 electrodes with a
dipole–dipole electrode configuration and minimum elec-
trode offset equal to 5 m. Details about the survey and the
2D electrical resistivity method are available elsewhere
(Griffiths and Barker 1993; Loke and Barker 1996). In
addition, vertical electrical sounding (VES) has been con-
ducted close to one of the TEM stations and nearby to the
borehole (QAP2-1) to relate the responses of the geoelec-trical techniques (TEM and VES) to the different subsur-
face lithological units in the study area (Fig. 1). Moreover,
22 water samples were collected and chemically analyzed
during the summer of 2012. The results for nitrate (NO3)
concentration are presented here and serve as a useful
source indicator of anthropogenic groundwater contami-
nation (Fig. 4).
Results
Nitrate concentrations
The nitrate concentrations in 22 boreholes in Al-Quwy’yia
region ranged from 1 to 49 mg/L, with a mean and SD of
37.5 and 14.4 mg/L, respectively (Fig. 4). These results
contrast markedly with that for non-polluted groundwater,
where typical nitrate levels are much less than 1 mg/L.
Thus, the presence of nitrate in groundwaters at levels
greater than about 3 mg/L usually reflects the impact of
Fig. 4 Areal distribution of
nitrate concentrations (mg/L) of the Khuff aquifer
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human activities on well water quality. Nitrate concentra-
tions in groundwater [10 mg/L indicates significant con-
tamination that is usually of biogenic origin (Mueller et al.
1995). Based on the classification of approach of Madisonand Brunett (1985), 45.6 % of the samples in this study had
nitrate concentrations of \44.3 mg/L, while the nitrate
levels for 54.5 % of the samples were [44.3 mg/L
(Table 1). The spatial distribution of nitrate was interpo-
lated using the kriging method and the fitted variogram,
showed that the southeastern part of the study area gener-
ally had the lowest nitrate concentrations (Fig. 4). These
low concentration levels indicate a limited impact of
human activity on the fresh water flow from the western
and southwestern basement rocks. Higher concentrations of
nitrates ([44.3 mg/L) were dominant in the urban regions
and at dump sites. These higher values of nitrate were
related to the infiltration and seepage of sewage from septic
tanks and the dumping of waste water along the south-
eastern part of the urban area (Fig. 4). Thus, the concen-
tration profiles and distribution patterns for NO3 suggests
that the aquifer has been already affected by the infiltrationof pollutant chemicals from the surface.
Comparison of the VES model with the borehole logs
To understand the different responses of subsurface litho-
logical units in the resistivity model; the VES (B1) was
been carried out close to a borehole (QAP2-1) located near
a waste dump site (Fig. 1). In the lithological description
there was no indication of groundwater contamination in
this borehole. The uppermost alluvium has at least two
resistivity layers based on the moisture and lithological
contents of this zone (Fig. 5). It extends to a depth of about
7 m from the ground surface. The limestone layer of the
Khuff Formation also has two resistivity layers on the VES
model based on the clay contents and extends to a depth of
about 96 m. Indeed, it shows relatively low resistivity
values in comparison with the alluvium layer. Most of the
recorded groundwater was been defined in the Khuff
limestone layer. Then the basement complex was been
recorded at a depth of about 96 m and showed a high
resistivity characteristics in the VES model (Fig. 5).
Comparison of VES and TEM data sets
After recording the signatures of different lithological units
in the resistivity models, the VES and TEM data sets were
compared with the resultant models of the two data sets.
VES 13 and TEM 32 are an example of this comparison
(Fig. 6). There is good consistency between the two
inverted models as they represent the same subsurface
lithological units, but the measurements for each technique
were carried out differently. At shallow parts of the VES
model there are three layers, which are represented in the
Table 1 Nitrate concentrations in groundwater of the Khuff aquifer
Zone Nitrate
concentration
Sample
no.
Sample
%
Remarks
I \0.89 mg/L 0 0 Assumed to represent
natural background
concentrations
II 0.89–13.29
mg/L
2 9.1 Transitional;
concentrations that
may or may not
represent human
activities
III 13.29–44.29
mg/L
8 36.4 Indicates elevated
concentrations
resulting from
human activities
IV [44.29 mg/L 12 54.5 Exceeds maximum
concentration for
National Interim
Primary Drinking-
Water Regulations
Fig. 5 Comparison of VES B1
model with the borehole
(QAP2-1) lithological
succession: a the filed curve of
VES B1, b the inverted
resistivity-depth model (1) and
the equivalence models (2) in
comparison with the different
lithological units
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TEM model by only one layer. This relates to the dense
electrode separations at the small offsets in acquiring the
VES data and the late time of recording the TEM mea-
surements. In addition, the VES and TEM data sets have
almost the same response for the possible contaminated
zone of the limestone layer at 45 m depth from the ground
surface and extending to a depth of about 55 m (Fig. 6). It
is possible to trace the variations in the measured TEM data
Fig. 6 Comparison of VES 13
with TEM 32 conducted at the
same site: a the filed curves of
VES 13 and TEM 32,
(b) inverted resistivity-depth
models with the interpreted
lithological units
Fig. 7 Examples of themeasured data sets along the 1st
profile with the corresponding
inverted resistivity models:
a the measured field curves,
b the inverted resistivity-depth
models
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Underneath the surface layer the limestone layer is domi-
nant with relatively high resistivity values (25–80 X m).
However, inside this layer and close to the dump site
(nearby stations 48, 49 and 50 in the 1st profile and 55 in
the 2nd profile) the resistivity values decrease again
because of the effects on the polluted groundwater from the
dump site. Underneath the TEM stations (st. 50, 51 and 54
in the 1st profile and st. 51 and 54 in the 2nd profile) the
resistivity values increase ([150 X m) because of the
basement complex occurrence. On the other hand, there is
no evidence of subsurface contamination from the dump
site for the third profile (Fig. 8c) as indicated by the similar
resistivity values for the limestone rock. This profile is
rather far away from the dump site location.
Results for 2D ERT
2D ERT data were acquired along two profiles close to the
known dump site. The acquired data reflect relatively dense
sampling of the subsurface in comparison with the pro-cessed TEM profiles. To produce cross-sections showing
the subsurface resistivity distribution in a 2D manner,
RES2DINV software utilizing a smoothness constraint was
used. The calculated data were compared with the field
data and the resistivity model was updated based on the
difference between the observed and calculated data. This
procedure was continued until the calculated data matched
the actual measurements with an acceptable level of error.
There were two profiles under consideration with one being
parallel to the dump site, while the other was at the right
angles to the dump site. Inspection of the inverted 2D
electrical resistivity profiles acquired very close to the
dump site (Fig. 1) revealed many important features
(Fig. 9). The parallel profile was more affected by the
sewage water seepage in the subsurface producing a rela-
tively low resistivity for the limestone layer (Fig. 9a). The
thickness of the contaminated layer is more than 60 m and
this reduces in going away from the dump site as the
basement rocks start to be traced (Fig. 9b). The 2D ERT
profiles confirm the results of the TEM data sets and show
that the contaminated zones have low resistivity
characteristics.
Conclusion
The study area of Al-Quwy’yia is considered a promising
region for agricultural and industrial projects, but sustain-
able development of the area is compromised by pollutionof the limited groundwater resources. The rapid growth of
human activities in the study area is accompanied by the
contamination and overexploitation of the groundwater
resources. The study identified possible anthropogenic
processes affecting the groundwater quality of the Khuff
aquifer in the study area. The nature and spatial distribution
of the contamination observed in the aquifer indicated that
the main contamination sources came from seepage of
sewage water from septic tanks and dumping of waste
Fig. 9 2D ERT Profiles acquired close to the dump site: a parallel to the dump site, b at the right angle to the dump site
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water outside of the urban areas. The TEM and 2D ERT
techniques have been applied in an attempt to delineate the
contaminated areas and obtain responses for both data sets.
The measured data at contaminated sites exhibited rela-
tively low resistivity characteristics in comparison with the
unaffected areas. Therefore, as long as there are low
resistivity zones in the measured data sets, this is an indi-
cation of contaminated plumes in the groundwater. Suchphenomena have been confirmed using the calibration
process between the measured data sets and the lithological
units in the study area. The TEM and 2D ERT both gave a
clear indication of the contaminated areas, but at different
spatial resolutions. The TEM technique resolved the con-
taminated zone vertically, whereas the 2D ERT defined its
lateral extension along the measured profiles. It is recom-
mended to apply both TEM and 2D ERT techniques at high
density around potential the dump sites to enable a clear
mapping of the direction of the contamination plume.
Demonstration of complete chemical analysis of the col-
lected water samples at Al-Quwy’yia area will be thesubject for future research work.
Acknowledgments This work was supported financially by the
National Plan for Science, Technology and Innovation (NPST) pro-
gram, King Saud University, Saudi Arabia (Project No. 09-ENV836-
02).
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